Synthetic polyisoprenes and a process for their preparation

ABSTRACT

The present invention provides synthetic polyisoprenes having a high cis-1,4 linkage content and a process for their preparation. The synthetic polyisoprenes according to the invention have a cis-1,4 linkage content, measured by carbon-13 nuclear magnetic resonance and/or medium-wave infrared radiation analysis, which lies within a range from 99.0% to 99.6%. The process for the preparation of these synthetic polyisoprenes involve polymerizing, at a temperature of 0° C. or lower, isoprene in the presence of a catalytic system based on: a) a conjugated diene monomer, b) an organic phosphoric acid salt of one or more rare earth, c) an alkylaluminium alkylating agent of the formula AlR3 or HAlR2, and d) a halogen donor consisting of an alkylaluminium halide, wherein the salt is suspended in at least one inert, saturated aliphatic or alicyclic hydrocarbon solvent and the alkylating agent: rare earth salt molar ratio ranges from 1 to 5.

BACKGROUND OF THE INVENTION

[0001] The present application is a continuation of PCT/EP01/12489,filed on Oct. 29, 2001, now WO 02/38635 (published in French). Thepresent invention relates to synthetic polyisoprenes having a highcis-1,4 linkage content and a process for their preparation.

[0002] It is known that polyisoprenes having a high cis-1,4 linkagecontent may be prepared using catalytic systems based on:

[0003] a rare earth salt in solution in a hydrocarbon solvent,

[0004] an alkylating agent of this salt formed of an alkylaluminium, and

[0005] a halide of an alkylaluminium.

[0006] For example, the document “Report of the Academy of Sciences ofthe U.S.S.R., volume 234, No. 5, 1977 (Y. B. Monakov, Y. R. Bieshev, A.A. Berg, S. R. Rafikov)” shows that isoprene may be polymerized at atemperature of between 20° C. and 50° C. using a catalytic systemcomprising:

[0007] a bis(2-ethylhexyl)phosphoric acid salt of neodymium orpraseodymium, as rare earth salt, in solution in toluene,

[0008] triisobutylaluminium as alkylating agent, in a molar ratio(alkylating agent/rare earth salt) of 20, and

[0009] diethylaluminium chloride as halide of an alkylaluminium.

[0010] Mention may also be made of the document “Proceedings ofChina—U.S. Bilateral Symposium on Polymer Chemistry and Physics, SciencePress, pp. 382-398, 1981 (O. Jun, W. Fosong, S. Zhiquan)”. This documentteaches the use of a bis(2-ethylhexyl)phosphoric acid salt of neodymium,in association with triethylaluminium or triisobutylaluminium, and analkylaluminium halide of the formula Al₂(C₂H₅)₃Cl₃. The polyisopreneswhich are obtained by means of such a catalytic system have a cis-1,4linkage content which varies from 94.2% to 94.7% (see Tables 4 and 6,pp. 386 and 387).

[0011] This document also mentions the use of catalytic systems basedon:

[0012] rare earth naphthenate, wherein the cis-1,4 linkage content ofthe obtained polyisoprenes is between 93.6% and 96.0%; and

[0013] rare earth trichloride (catalytic system of formulaLnCl3-C2H5OH—Al(C2H5)3), wherein the cis-1,4 linkage content of theobtained polyisoprenes being between 94.1% and 98.0% (this content of98% being achieved using ytterbium as rare earth, see Table 12 p. 391).

[0014] In the majority of cases, the microstructure of the polyisopreneis determined by the technique of medium-wave infrared radiationanalysis (abbreviated to MIR) in accordance with the method developed byCiampelli et al (F. Ciampelli, D. Moreno, M. Cambini, Makromol. Chem.,1963, 61, 250-253). It will be noted that this method, which is basedsolely on calculations made in the infrared range, does not alwaysprovide results of satisfactory accuracy when used in isolation.

[0015] U.S. Pat. No. 5,859,156 describes a process for the preparationof polyisoprenes by means of a catalytic system based on titaniumtetrachloride, an organoaluminium and an ether. The maximum cis-1,4linkage content of the polyisoprenes obtained, measured by carbon-13nuclear magnetic resonance (¹³C-NMR), was 98.0% (see example 2, column27, the content of trans-1,4 and 3,4 linkages then being 1.0% each).

SUMMARY OF THE INVENTION

[0016] The object of the present invention is to provide novel syntheticpolyisoprenes and a process for their preparation, said polyisopreneshaving a cis-1,4 linkage content which are distinctly greater than thoseobtained to date.

[0017] The present invention is based on the unexpected discovery that acatalytic system of the “preformed” type based on at least:

[0018] a conjugated diene monomer,

[0019] an organic phosphoric acid salt of a rare earth metal (a metalwith an atomic number between 57 and 71 in Mendeleev's periodic table),said salt being in suspension in at least one inert, saturated aliphaticor alicyclic hydrocarbon solvent,

[0020] an alkylating agent consisting of an alkylaluminium of formulaAlR₃ or HAlR₂, and

[0021] a halogen donor consisting of an alkylaluminium halide,

[0022] wherein the alkylating agent: rare earth salt molar ratio variesfrom 1 to 5, makes it possible to polymerize isoprene with satisfactoryactivity at polymerization temperatures which are less than or equal to0° C., and to obtain at these low temperatures polyisoprenes having acis-1,4 linkage content, measured using the technique of carbon-13nuclear magnetic resonance and/or the technique of medium-wave infraredradiation analysis, of greater than 99.0%.

[0023] The phrase “based on” used to define the constituents of thecatalytic system is taken to mean the mixture and/or reaction product ofthese constituents. Also, the phrase “a rare earth metal” is intended tomean “one or more rare earth metals.”

[0024] This catalytic system according to the invention makes itpossible to obtain polyisoprenes having a cis-1,4 linkage content,measured by one or the other of these techniques, of between 99.0% and99.6%, including 99.6%.

[0025] The polymerization operations can be carried out in the presenceor absence of an inert hydrocarbon solvent.

[0026] Advantageously, at polymerization temperatures of from −55° C. to−20° C., the catalytic system makes it possible to obtain polyisopreneshaving a cis-1,4 linkage content, measured by one and/or the other ofthe aforementioned techniques, of equal to or greater than 99.3%,preferably from 99.3% to 99.6%.

[0027] Even more advantageously, at polymerization temperatures of from−55° C. to −40° C., the catalytic system makes it possible to obtainpolyisoprenes having a cis-1,4 linkage content, measured by one and/orthe other of the aforementioned techniques, are equal to or greater than99.5%, preferably equal to 99.6%.

[0028] It will be noted that the cis-1,4 linkage content values, whichare very close to the value of 100%, that characterizes natural rubber,have never really been achieved to date. The range of cis-1,4 linkagecontent, measured in accordance with the present invention takes intoaccount measurements established by means of either medium-wave infraredradiation analysis after calibration of the polyisoprene sample, whichis effected within the scope of ¹³C-NMR analysis, or ¹³C-NMR analysis.The measurement obtained by one of these techniques can be confirmed bythat of the other (disregarding the inaccuracy of measurement of ±0.1%,which is inherent in each of these two techniques). The accuracy ofthese cis-1,4 linkage content values is thus significantly increased,relative to those of the prior art.

[0029] In particular, ¹³C NMR analysis showed the total absence of 1,2linkages and trans-1,4 linkages in the polyisoprene prepared accordingto the invention.

[0030] Furthermore, the particularly high cis-1,4 linkage contentobtained for the polyisoprenes according to the invention is independentof the quantity of catalytic system used. Moreover, the polyisoprenesthus obtained have a high viscosity.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The catalytic systems according to the invention arecharacterized by an “alkylating agent: rare earth salt” molar ratio ofbetween 1 and 5, which is extremely low compared with the molar ratiosequal to or greater than 20 which have heretofore been used topolymerize isoprene.

[0032] 1,3-butadiene may be mentioned as a preferred conjugated dienemonomer used for “preforming” the catalytic system according to theinvention.

[0033] Other conjugated dienes that may be used are2-methyl-1,3-butadiene (or isoprene), 2,3-di(C1 to C5alkyl)-1,3-butadienes such as, for instance, 2,3-dimethyl-1,3-butadiene,2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene,2-methyl-3-isopropyl-1,3-butadiene, phenyl-1,3-butadiene,1,3-pentadiene, 2,4-hexadiene, or any other conjugated diene havingbetween 4 and 8 carbon atoms.

[0034] It will be noted that the “monomer: rare earth salt” molar rationmay have a value of from 25 to 50.

[0035] According to another characteristic of the invention, said rareearth salt consists of a non-hygroscopic powder having a slight tendencyto agglomerate at ambient temperature.

[0036] According to a preferred embodiment of the invention, the inerthydrocarbon solvent in which said rare earth salt is in suspension is alow molecular weight aliphatic or alicyclic solvent, such ascyclohexane, methylcyclohexane, n-heptane, or a mixture of thesesolvents.

[0037] According to another embodiment of the invention, the solventused to suspend the rare earth salt is a mixture of a high molecularweight aliphatic solvent comprising a paraffinic oil, for examplepetrolatum oil, and of low molecular weight, a solvent such as thosementioned above (for example methylcyclohexane).

[0038] This suspension is prepared by dispersive grinding of the rareearth salt in this paraffinic oil, so as to obtain a very fine andhomogenous suspension of said salt.

[0039] According to another characteristic of the invention, saidcatalytic system comprises the rare earth metal in a concentration equalor substantially equal to 0.02 mol/l.

[0040] According to a preferred example of embodiment of the invention,a tris[bis(2-ethylhexyl)phosphate] salt of the rare earth metal ormetals is used as salt.

[0041] Even more preferably, the rare earth salt is neodymiumtris[bis(2-ethylhexyl)phosphate].

[0042] Alkylating agents that can be used in the catalytic systemaccording to the invention, include alkylaluminiums such as:

[0043] trialkylaluminiums, for example triisobutylaluminium, or

[0044] dialkylaluminium hydrides, for example diisobutylaluminiumhydride.

[0045] It will be noted that the alkylating agent is preferablydiisobutylaluminium hydride (hereinafter “DiBAH”).

[0046] Halogen donors that can be used in the catalytic system accordingto the invention included alkylaluminium halides, preferablydiethylaluminium chloride (hereinafter “DEAC”).

[0047] The “halogen donor: rare earth salt” molar ratio may have a valueof from 2.6 to 3.

[0048] According to the invention, the process for the preparation ofsaid catalytic system comprises:

[0049] (i) producing a suspension of said rare earth salt in saidsolvent,

[0050] (ii) adding said conjugated diene monomer to the suspension,

[0051] (iii) adding said alkylating agent to the suspension comprisingsaid monomer to obtain an alkylated salt, and

[0052] (iv) adding said halogen donor to the alkylated salt.

[0053] The aforementioned characteristics of the present invention, aswell as others, will be better understood on reading the followingdescription of several examples of embodiment of the invention, whichare given by way of illustration and not of limitation.

EXAMPLE I

[0054] Preparation of Catalytic Systems of the Invention:

[0055] 1) Synthesis of an Organic Phosphate Salt of Neodymium of theInvention:

[0056] A plurality of tests were carried out for synthesis of this salt.The same synthesis method, which is described in detail below, was usedfor each of these tests.

[0057] a) Synthesis of an Aqueous Solution of Neodymium NdCl₃, 6H₂O:

[0058] 96 g of Nd₂O₃ (sold by RHODIA), which has been determined bycomplexation analysis to have an Nd content of 85.3% (theoretical value85.7%), thereby amounting to 0.57 mol of Nd, was weighed out into a“tall” form 600 ml beaker.

[0059] 80 ml of demineralized water were added. Under a fume hood, 150ml of 36 wt. % concentrated HCl (d=1.18), namely 1.75 mol of HCl (molarratio HCl:Nd=1.75:0.57=3.07), were slowly added at ambient temperaturewhile the mixture was stirred with a magnetic stirrer.

[0060] The reaction Nd₂O₃+6 HCl+9 H₂O→2 NdCl₃, 6H₂O is highlyexothermic.

[0061] Once all the hydrochloric acid has been added, the solution wasraised to boiling while being stirred with a magnetic stirrer. Theaqueous NdCl₃ solution was clear and mauve in color. No insolubleproduct (Nd₂O₃) remained.

[0062] This solution was then evaporated until a volume of 130 mlremained in the beaker. The NdCl₃, 6H₂O was highly concentrated (itcrystallized at ambient temperature).

[0063] The concentrated solution of NdCl₃ was then poured into a 10liter drum containing 4500 ml of demineralized water at ambienttemperature, while the mixture was stirred (using a motor with an anchoragitator).

[0064] The pH of the solution, measured at 25° C., was close to 4.

[0065] 1500 ml of technical grade acetone were then added to thesolution. No insoluble product remained and the resultant solution waspink in color.

[0066] b) Synthesis of an Organic Sodium Phosphate of Formula[RO]₂P(O)ONa (R=2-ethylhexyl):

[0067] 68 g, or 1.70 mol, of NaOH flakes were dissolved in a 5 literbeaker containing 1500 ml of demineralized water. 554 g of an organicphosphoric acid (bis(2-ethylhexyl)phosphoric acid, listed in the“Aldrich” catalogue under number 23,782-5), namely 1.72 mol of thisacid, were dissolved in another 3 liter beaker containing 500 ml ofacetone. The molar ratio NaOH: organic phosphoric acid was 1.70:1.72 or0.99.

[0068] At ambient temperature and while stirring the mixture by handwith a glass stirrer, the solution of said organic phosphoric acid waspoured into the NaOH solution. The reaction is as follows:

[RO]₂P(O)OH+NaOH→[RO]₂P(O)ONa+H₂O.

[0069] The reaction is slightly exothermic and a homogeneous solution ofa yellowish color was obtained. The pH of the solution, measured at 25°C., was close to 7.

[0070] c) Synthesis of a Phosphated Neodymium Salt of the Formula[[RO]₂P(O)O]₃Nd:

[0071] At ambient temperature and while the mixture was being vigorouslystirred (motor with anchor agitator), the organic Na phosphate saltobtained in paragraph b) above was poured into the aqueous solution ofNdCl₃, 6H₂O obtained in paragraph a) above.

[0072] A very fine white precipitate formed immediately. Stirring of theresultant mixture was continued for 30 minutes once all the organic Naphosphate had been added (in a molar ratio(RO)₂P(O)ONa:NdCl₃=1.70:0.57=2.98). The reaction is as follows:

3[RO]₂P(O)ONa+NdCl₃, 6H₂O→Nd[OP(O)[OR]₂]₃+3 NaCl+6 H₂O.

[0073] The resultant phosphated neodymium salt was recovered and washedin a centrifuge equipped with a “sock”.

[0074] The pH of the mother liquors was between 3 and 4 at 25° C. Thesemother liquors were colorless and clear.

[0075] The salt obtained was divided into two samples, then each samplewas washed with an acetone/demineralized water mixture, performing thewashing cycle described below three times in order to remove all thechlorides.

[0076] Each washing cycle was performed in a 10 liter plastic bucketinitially containing 2 liters of acetone. Each sample was thenhomogenized with the acetone using an “Ultra-Turrax” homogenizer forapproximately 1 minute in order to obtain a milky solution.

[0077] 4 liters of demineralized water were then added to the bucket andthe resultant mixture was homogenized for 3 minutes using the samehomogenizer.

[0078] The resultant mixture was centrifuged and the phosphatedneodymium salt was recovered in the “sock”.

[0079] The qualitative analytic test for chlorides was virtuallynegative for the final washing water (the reaction is as follows:

NaCl+AgNO₃ (HNO₃ medium)→AgCl↓+NaNO₃).

[0080] The neodymium salt washed in this manner is dried in an ovenunder a vacuum and with air-flow for approximately 80 hours.

[0081] The final yield for each of the synthesis tests performed wasbetween 95% and 98%, depending upon the losses arising during washing.In each case, approximately 600 g of dry phosphated neodymium salt wasobtained.

[0082] The mass contents of neodymium, determined by complexometry, werebetween 12.9% and 13.0% (for a theoretical contentτ=[144.24/1108.50]×100=13.01%, where 144.24 g/mol=molar mass of theneodymium).

[0083] 2) Synthesis of “Preformed” Catalytic Systems According to theInvention:

[0084] a) Composition of Catalytic Systems According to the Invention:

[0085] Each of these systems comprised a phosphated neodymium salt assynthesized in accordance with paragraph 1) above, which is insuspension in an inert low molecular weight hydrocarbon solvent(consisting of cyclohexane, hereinafter to “CH,” or methylcyclohexane,hereinafter “MCH”).

[0086] These catalytic systems were characterized by the followingrelative molar ratios, relative to the neodymium salt:

[0087] Nd salt:butadiene (“Bd”):DiBAH:DEAC=1:50:1.8 to 4:2.6 or 3.

[0088] b) Process for Synthesising these Catalytic Systems:

[0089] First Stage:

[0090] In order to obtain these catalytic systems, 15.6 g of theneodymium salt, in powder form, was poured into a 1 liter reactor fromwhich the impurities had been previously removed. This salt was thensubjected to nitrogen bubbling from the bottom of the reactor, for aperiod of 15 min.

[0091] Second Stage:

[0092] 90% (mass fraction) of the solvent mentioned in paragraph a)above was introduced into the reactor containing the neodymium salt.

[0093] When the solvent was cyclohexane, the duration of contact withthe neodymium salt varied from 2 hours to 4 hours, and the temperaturevaried from 30° C. to 60° C. When the solvent was methylcyclohexane, theduration of contact with the neodymium salt was 30 min., and thetemperature of contact was 30° C.

[0094] Third Stage:

[0095] Butadiene was then introduced into the reactor (in thesalt:butadiene molar ratio of 1:50 mentioned in paragraph a) above), ata temperature of 30° C., for “preforming” each catalytic system.

[0096] Fourth Stage:

[0097] Diisobutylaluminium hydride (DiBAH) was then introduced into thereactor as alkylating agent for the neodymium salt in a concentration ofapproximately 1 M, together with a quantity of the solvent mentionedabove in the second stage corresponding to a mass fraction of 5% of theentire solvent. The duration of the alkylation was 15 min. and thetemperature of the alkylation reaction was 30° C.

[0098] Fifth Stage:

[0099] Diethylaluminium chloride (DEAC) was then introduced into thereactor as the halogen donor in a concentration of approximately 1 M,together with a quantity of the solvent mentioned above in the secondstage corresponding to the remaining mass fraction of 5% of the entiresolvent. The temperature of the reaction medium adjusted to 60° C.

[0100] Sixth Stage:

[0101] The resultant mixture was then “preferred” (or aged) bymaintaining this temperature of 60° C. for a period of 2 hours to 4hours.

[0102] Seventh Stage:

[0103] In this manner, approximately 700 ml of a solution of catalyticsystem was obtained. The reactor was emptied and this solution wastransferred to a 750 ml “Steinie” bottle, which had been previouslywashed, dried and subjected to nitrogen bubbling.

[0104] Finally the catalytic solution was stored under a nitrogenatmosphere in a freezer, at a temperature of −15° C.

[0105] Table I provides a summary of the catalytic systems that wereprepared. TABLE I Solvation Preforming Catalytic (solvent/ Bd/Nd Al/NdAlkylation Cl/Nd with DEAC systems duration/T) (mol) (mol) Duration/T(mol) Duration/T System 1 CH 50 4 15 min./30° C. 3 4 h, 60° C. 2 h, 30°C. System 2 CH 50 3 15 min./30° C. 3 2 h, 60° C. 4 h, 60° C. System 3MCH 50 3 15 min./30° C. 3 4 h, 60° C. 30 min., 30° C. System 4 MCH 501.8 15 min./30° C. 3 2 h, 60° C. 30 min., 30° C. System 5 MCH 50 1.8 15min./30° C. 2.6 2 h, 60° C. 30 min., 30° C. System 6 CH 50 4 15 min./30°C. 3 2 h, 60° C. 2 h, 60° C.

EXAMPLE II

[0106] Polymerization of Isoprene by Means of the Catalytic Systems ofExample I:

[0107] 1) Examples of Polymerization of Isoprene at a Temperature of−15° C. by Means of Catalytic System 1:

[0108] a) Polymerization Process Used:

[0109] A 250 ml “Steinie” bottle was used as the polymerization reactor.Each polymerization reaction was carried out either under staticconditions in a freezer (bottle placed in a bath of glycol), ordynamically (by subjecting the bottle to agitation in a tank of glycol).

[0110] A steam-cracked C5 naphtha fraction was used, with the aim ofextracting isoprene having a purity close to 100% therefrom. To thisend, conventional laboratory purification was carried out, comprising:

[0111] distillation of this C5 fraction over maleic anhydride toeliminate any residual cyclopentadiene, followed by

[0112] passing through a column of alumina to eliminate the polarimpurities, and

[0113] nitrogen bubbling for 20 min., immediately prior to thepolymerization reaction.

[0114] The mass fraction of isoprene extracted from this C5 fraction wasdetermined to be 99.2% by gas phase chromatography (“GPC”).

[0115] Each isoprene polymerization reaction (10 g per bottle) wascarried out in cyclohexane at −15° C., under an inert nitrogenatmosphere, with a solvent:monomer mass ratio (S:M) of 9.

[0116] In the various examples of polymerization, the neodymium catalystbase was varied from 150 μmol to 500 μmol per 100 g of monomer (quantityof neodymium expressed in μMcm hereafter). The tightness of the bottlewas ensured by a “septum/open-top seal” assembly which permitted theinjection of the catalytic system by means of a syringe.

[0117] At the end of polymerization, while adding 100 ml of additionalsolvent to fluidify the medium, acetylacetone was added (1 ml of asolution of a concentration of 1 M in cyclohexane) to stop the reactionand N-1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine (abbreviated to 6PPD) as a protection agent (in a volume of 2 ml at a concentration of 10g/l in cyclohexane, or a mass of 0.02 g).

[0118] The polyisoprene was extracted from each polymer solution bysteam stripping for 30 minutes, in the presence of calcium tamolate(using 2 ml of tamol and 50 ml of CaCl₂ at 30 g/l). Then each extractedsolution was dried for approximately 18 hours in an oven at 50° C. undera vacuum (at a pressure of 200 mm Hg), under a gentle stream of nitrogenfor approximately 72 hours.

[0119] b) Results Obtained:

[0120] The conversion rate of isoprene into polyisoprene as a functionof the reaction time was measured to describe the polymerizationkinetics.

[0121] The inherent viscosity η_(inh) at 0.1 g/dl in toluene and theMooney viscosity ML(1+4) (measured in accordance with ASTM StandardD-1646) characterize the macrostructure of each polyisoprene obtained.

[0122] To determine the microstructure of the polyisoprenes, ¹³C-NMR(absolute method of great accuracy) and MIR (medium-wave infrared)analyses were used, as specified in “Measurements and Tests” below.These techniques have made it possible to establish, to within aninaccuracy of 0.1%, the cis-1,4 and 3,4 linkage content (¹³C-NMRanalysis having demonstrated the absence of trans-1,4 or 1,2 linkages).

[0123] The MIR technique provided great accuracy in determining thecontent of 3,4 units, owing to the fact that it used the samples ofpolyisoprene having previously been calibrated for the ¹³C-NMR analysis.

[0124] Table II sets forth the operating conditions followed for eachpolymerization and the macrostructural and microstructuralcharacteristics of each polyisoprene obtained. TABLE II Characteristicsof the polyisoprenes Polymerization conditions Content Content QuantityReaction Conversion Inherent of cis-1,4 of type of of Nd time rateviscosity by cis-1,4 Tests reactor (μMcm) (hours) (%) (dl/g) ML(1 + 4)¹³C-NMR by MIR A Static 500 64 100 4.9 79 99.2 99.1 400 64 100 5.3 87 —— B Static 400 64 100 5.2 82 — — stirred 400 64 100 — 83 — — C Static300 64 100 6.0 93 99.1 99.0 D Static 200 64 100 7.2 — 99.2 — 150 64 1008.5 — 99.2 — E Static 150 64 100 8.6 — — — stirred 150 64 100 8.9 — — —F Static 150 64 100 — — — 99.1 47 98 8.0 — — 99.2 38 94 — — — 99.1 22 607.4 — — 99.2 14 42 6.8 — — 99.2

[0125] Table II shows that at a polymerization temperature of −15° C.,catalytic system 1 according to the invention made it possible to obtainpolyisoprenes having a content of cis in a range from 99.0% to 99.2%with satisfactory activity, whatever the quantity of catalytic base usedand whatever the conversion rate achieved for a given quantity ofcatalytic base.

[0126] The results also show that stirring had no influence on themacrostructure of the polyisoprenes obtained. Although the reactionrates were relatively low, the activity of catalytic system 1 was notadversely affected and made it possible to achieve 100% conversion, evenfor final polyisoprenes having a high viscosity.

[0127] 2) Examples of Polymerization of Isoprene at a Temperature of−25° C. by Means of the Aforementioned Catalytic Systems 2, 3, 4 and 5:

[0128] a) Polymerization Process Used:

[0129] A 750 ml “Steinie” bottle was used as the polymerization reactor,and all the polymerization reactions of the isoprene were implementedstatically in a freezer at −25° C. (bottle in a bath of glycol).

[0130] The quality of the isoprene was as described in paragraph 1)above. The conversion rate was 100% in all cases (for at least 64 hoursof reaction time).

[0131] Each polymerization was effected as indicated in paragraph 1),except that a plurality of polymerization solvents and solvent:isoprenemonomer (S:M) mass ratios were tested, for the tests carried out.

[0132] Because of the viscosity of the polyisoprene solutions obtained,solvent was added thereto, before adding the stoppage and protectionagents mentioned in paragraph 1).

[0133] The polyisoprene was then extracted from each polyisoprenesolution thus “fluidified”, then each extracted solution was dried, allin the manner described in paragraph 1).

[0134] b) Results Obtained:

[0135] Table III shows the results obtained (refer to paragraph 1) abovefor the measurement conditions). TABLE III Polymerization conditionsPolyisoprenes obtained Quantity Content Quantity of of cis-1,4 ContentCat. Polymerization of Nd isoprene Ratio by of cis-1,4 Tests syst.solvent (μMcm) (g) S/M ML(1 + 4) ¹³C-NMR by MIR G 2 none 500 50 0 52 —99.3 1000 50 0 35 — 99.3 n-pentane 1000 50 1 30 99.4 99.4 cyclopentane1000 50 1 40 — 99.4 CH 1000 50 1 34 — 99.3 MCH 1000 50 1 39 — 99.3 H 3MCH 1000 36 9 69 — 99.4 700 36 9 79 — 99.4 I 3 MCH 700 72 5 80 — 99.5 J3 MCH 700 72 5 79 — 99.5 K 3 MCH 700 72 5 81 — 99.4 L 3 MCH 700 72 5 84— 99.4 4 MCH 700 72 5 89 — 99.4 5 MCH 700 72 5 97 — 99.3

[0136] These results show that, at a temperature kept at −25° C.,catalytic systems 2 to 5 according to the invention made it possible toobtain polyisoprenes having cis-1,4 linkage contents which are onaverage equal to 99.4% with satisfactory activity.

[0137] Insofar as the polymerization temperature remained constant, thepresence or absence of solvent, the nature of this solvent (aliphatic oralicyclic) and the quantity of solvent had no effect on cis-1,4 linkagecontent.

[0138] As far as the catalytic systems used are concerned, it will benoted that the cis-1,4 linkage content was independent of the molarratios DiBAH/Nd and DEAC/Nd.

[0139] As far as the macrostructure characteristics of the polyisoprenesobtained are concerned (measurements taken by means of the SECtechnique, see “Measurements and Tests” below), the polyisopreneobtained for test G (catalytic system 2) with a quantity of Nd of 500μMcm had:

[0140] a number-average molecular weight Mn of 338,475 g/mol, and

[0141] a polymolecularity index Ip of 2.389.

[0142] As for the polyisoprene obtained for test K (catalytic system 3),it had:

[0143] a number-average molecular weight Mn of 423,472 g/mol, and

[0144] a polymolecularity index Ip of 2.483.

[0145] 3) Examples of Polymerization of Isoprene at a Temperature of−45° C. by Means of Catalytic System 3:

[0146] a) Polymerization Process Used:

[0147] The same polymerization conditions as those set forth inparagraph 2) above were used, except that the polymerization temperaturewas kept at −45° C. (instead of −25° C.).

[0148] b) Results Obtained:

[0149] Table IV shows the results obtained (reference will be made toparagraph 1) above for the measurement conditions). TABLE IVPolymerization conditions Polyisoprenes Quantity Content Quantity ofReaction Inherent of cis-1,4 Cat. of Nd isoprene Ratio time Conversionviscosity by MIR Test syst. (μMcm) (g) solvent S/M (h) rate (%) (dl/g)(%) M 3 700 72 MCH 5 144 14 6.3 99.6 1500 72 MCH 5 144 22 5.4 99.6

[0150] These results show that catalytic system 3 according to theinvention had a sufficient activity to polymerize isoprene at atemperature kept at −45° C., despite the reduced reaction rate which itprovided at this very low temperature.

[0151] It will be noted that the polyisoprenes thus obtained each have acis-1,4 linkage content of 99.6%, which is an extremely high amount.

[0152] 4) Examples of Polymerization of Isoprene at a Temperature of 0°C. by Means of Catalytic Systems 5 and 6:

[0153] a) Polymerization Process Used:

[0154] The same polymerization conditions as those set forth inparagraph 1) above (250 ml “Steinie” bottle with 10 g isoprene perbottle) were used, except that the polymerization temperature was keptat 0° C. and that the polymerization was implemented with stirring in atank of glycol.

[0155] b) Results Obtained:

[0156] Table V shows the results obtained (reference will be made toparagraph 1) for the measurement conditions). TABLE V Polyisoprenesobtained Polymerization conditions Content Content Quantity ConversionInherent of cis-1,4 of cis-1,4 Cat. Ratio of Nd Reaction rate viscosity(%) by (%) by Tests syst. S/M (μMcm) time (h) (%) (dl/g) ML(1 + 4)¹³C-NMR MIR N 6 9 130 48 100 — 97 99.1 — O 5 9 300 18 100 7.6 97 — 99.0700 18 100 5.8 84 — 99.1 P 5 9 700 0.25 15 — — — — 1.5 50 — — — — 2 604.9 — — 99.1 18 100 6.0 86 — 99.0 Q 5 9 700 18 100 — 85 — 99.0 7 700 18100 — 86 — 99.0

[0157] These results show that at a temperature of 0° C., catalyticsystems 5 and 6 according to the invention made it possible to obtainpolyisoprenes having a cis-1,4 linkage content within a range from 99.0%to 99.1% with satisfactory activity.

[0158] For a ratio of polymerization solvent:monomer(cyclohexane:isoprene) equal to 9 (namely 10% concentration), it will benoted that the polyisoprenes obtained with catalytic system 5 accordingto the invention had, after 18 hours (100% conversion), a high,reproducible Mooney viscosity of approximately 85.

[0159] As far as the macrostructure characteristics of the polyisoprenesobtained are concerned (measurements taken by means of the SECtechnique, see “Measurements and Tests” below), the polyisopreneobtained for test N (catalytic system 6) had:

[0160] a number-average molecular weight Mn of 930,299 g/mol, and

[0161] a polymolecularity index Ip of 2.46

MEASUREMENTS AND TESTS DETERMINATION OF THE MICROSTRUCTURE OF THEPOLYISOPRENES

[0162] 1) By carbon 13 Nuclear Magnetic Resonance Analysis (¹³C NMRanalysis):

[0163] a) Sample Preparation:

[0164] 2 g of polyisoprene were extracted in refluxing acetone for 8hours. The extracted polyisoprene was then dried at ambient temperatureunder a vacuum for 24 hours. This dried polyisoprene was thenredissolved in chloroform. The polyisoprene solution was filtered andthe solvent removed in a rotary evaporator for 4 hours (bath temperatureis 40° C.).

[0165] For the purposes of the analysis, approximately 600 mg the ofpolyisoprene prepared in this manner were solubilized in CDCl₃ (2 ml)directly in a ¹³C NMR tube.

[0166] b) Characteristics of the Apparatus:

[0167] Spectrophotometer sold under the name “BRUKER AM250”.

[0168] Resonance frequency (SFO)=62.9 MHz.

[0169] Pulse program: INVGATE.AU (suppression of “NOE” effect forquantitative analysis of ¹³C by NMR).

[0170] Pulse duration: 9 μs (90°).

[0171] Relaxation time: 10 s.

[0172] Cumulative number of scans (NS)=8192.

[0173] c) Assignment of Spectrum_Peaks:

[0174] Peaks were identified following:

[0175] Quang Tho Pham, R. Petiaud, H. Waton, M. F. Llauro Darricades,“Proton and NMR Spectra of Polymers”, 1991, Penton Press.

[0176] d) Integration Method:

[0177] No 1,2-structural units detected.

[0178] The ratio between 3,4-and 1,4-contents was determined by means ofthe ethylenic carbons. The content of trans-1,4 and cis-1,4 linkages inthe polyisoprene was calculated from the aliphatic carbons.

[0179] 2) By Mid-Infrared (MIR) Analysis:

[0180] a) Sample Preparation:

[0181] The polyisoprene as prepared in paragraph 1) above was used forthis infrared analysis, while for NMR the sample was extracted withacetone and then dried in an oven.

[0182] A polyisoprene solution of exactly 10 g/l in CCl₄ was analyzedusing a KBr cell with a pathlength of 0.2 mm.

[0183] b) Apparatus:

[0184] Spectrophotometer sold under the name “BRUKER IFS88”.

[0185] Recording conditions:

[0186] beam opening: maximum;

[0187] resolution: 2 cm⁻¹;

[0188] moving mirror speed: 0.639 cm.s⁻¹;

[0189] detector: DTGS;

[0190] accumulations: 64 scans;

[0191] purge time: 3 min;

[0192] spectral window: 4000 to 400 cm⁻¹;

[0193] transmission spectra recorded;

[0194] reference: CCl₄ solvent.

[0195] Spectrum processing:

[0196] transfer to microcomputer;

[0197] processing with “OPUS” software from “BRUKER”.

[0198] c) Assignment of Spectrum Peaks:

[0199] Spectral studies and the contents of the following documents madeit possible to determine the characteristic bands of the various linkagemodes:

[0200] Y. Tanaka, Y. Takeuchi, M. Kobayashi, H. Tadokoro, Journal ofPolymer Science, Part A-2, 1971, 9(1), 43-57.

[0201] J. P. Kistel, G. Friedman, B. Kaempf, Bulletin de la SociétéChimique de France, 1967, no. 12.

[0202] F. Asssioma, J. Marchal, C. R. Acad. Sc. Paris, Ser C, 1968,266(22), 1563-6 and Ser D, 1968, 266(6), 369-72.

[0203] T. F. Banigan, A. J. Verbiscar, T. A. Oda, Rubber Chemistry andTechnology, 1982, 55(2), 407-15.

[0204] The 3-4 conformation exhibited two characteristic bands:

[0205] a high intensity band at 880 cm⁻¹ corresponding to theout-of-plane deformation vibrations (δ C—H) of the terminal hydrogens ofthe vinyl group (═CH₂) and

[0206] a band at 3070 cm⁻¹ corresponding to the ν C—H stretching of thissame group (═CH₂).

[0207] The cis-1,4 conformation had a characteristic band around 3030cm⁻. This band corresponds to the ν C—H stretching vibrations of the ═CHgroup.

[0208] The band corresponding to the symmetrical deformation vibrationsof the methyl groups (δ CH₃) was a complex band incorporating all threeconformations. Absorption corresponding to the δ CH₃ of the trans-1,4conformation was at its maximum around 1385 cm⁻¹; this was a shoulder ofthe band.

[0209] d) Integration Method:

[0210] The cis-3,4 and 1,4 bands were integrated by the tangential areamethod.

[0211] The 1,4-trans absorption maximum was located on the shoulder ofthe intense δ CH₃ band. The most suitable method in this case was tomeasure the height of the band using the tangent of the δ CH₃ band asthe baseline.

[0212] e) Calibration Curves:

[0213] Statement of Beer-Lambert law:

Do(ν or δ)=ε(ν or δ) e c

[0214] where:

[0215] Do(ν or δ)=optical density of the band ν or δ;

[0216] ε(ν or δ)=molar extinction coefficient of the analyte responsiblefor the band ν or δ;

[0217] c=molar concentration of the analyte; and

[0218] e=sample thickness.

[0219] Commercial polyisoprenes (sold as “IR305”, “NATSYN 2200” and“SKI-3S”), a polyisoprene synthesized in the laboratory (MC78) andnatural rubber (NR) were used as standards. Compared at isoconcentration(solutions), the law may thus be written:

[0220] Dx=K X

[0221] where:

[0222] Dx=integration value of the band corresponding to structural unitX,

[0223] X=content of structural unit X in the rubber (determined by ¹³CNMR), and

[0224] K=calibration constant.

[0225] Calibration curves Dx=f(X) may thus be plotted for each of thestructural units.

DETERMINATION OF THE DISTRIBUTION OF MOLECULAR WEIGHTS OF THE ELASTOMERSOBTAINED BY SIZE EXCLUSION CHROMATOGRAPHY (SEC)

[0226] a) Measurement Principle:

[0227] SEC (size exclusion chromatography) makes it possible physicallyto separate macromolecules by their size in the swollen state in columnsfilled with a porous stationary phase. The macromolecules are separatedby their hydrodynamic volume, the bulkiest being eluted first.

[0228] Although not an absolute method, SEC does enable an assessment tobe made of the molecular weight distribution of a polymer. On the basisof commercially available standards, the various number-average (Mn) andweight-average (Mw) molecular weights may be determined and thepolydispersity index calculated (IP=Mw/Mn).

[0229] b) Preparation of the Polymer:

[0230] The polymer sample was not subjected to any particular treatmentprior to analysis, but is simply solubilized in tetrahydrofuran at aconcentration of approximately 1 g/l.

[0231] c) SEC Analysis:

[0232] The apparatus used was a “WATERS model 150C” chromatograph. Theelution solvent is tetrahydrofuran, the flow rate 0.7 ml/min, thetemperature of the system 35° C. and the duration of analysis 90 min. Aset of four columns was used in series, the columns having thecommercial names “SHODEX KS807”, “WATERS type STYRAGEL HMW7” and two“WATERS STYRAGEL MHW6E”.

[0233] The volume of polymer sample solution injected was 100 μl. Thedetector is a “WATERS model R132X” differential refractometer and thechromatographic data processing software was “WATERS MILLENNIUM”(version 3.00).

I claim:
 1. A synthetic polyisoprene, said polyisoprene having a cis-1,4linkage content which is greater than 99.0%, wherein the cis 1,4 linkagecontent is measured by carbon-13 nuclear magnetic resonance and/ormedium-wave infrared radiation analysis.
 2. The synthetic polyisopreneaccording to claim 1, having a cis 1,4 linkage content which is equal toor greater than 99.3%.
 3. The synthetic polyisoprene according to claim2, having a cis-1,4 linkage content which is equal to or greater than99.5%.
 4. The synthetic polyisoprene according to claim 3, having acis-1,4 linkage content which is equal to 99.6%.
 5. A process for thepreparation of a synthetic polyisoprene having a cis 1,4 linkage contentwhich is greater than 99.0%, comprising polymerizing isoprene in thepresence of a catalytic system, said catalytic system being based on a)a conjugated diene monomer, b) an organic phosphoric acid salt of a rareearth metal, c) an alkylating agent consisting of an alkyaluminum of theformula AlR₃ or HAl R₂, and d) a halogen donor consisting of analkylaluminum halide, wherein said salt is suspended in at least oneinert, saturated aliphatic or alicyclic hydrocarbon solvent and thealkylating agent:rare earth molar ratio ranges from 1 to 5, wherein saidpolymerization is carried out at a reaction temperature of less than orequal to 0° C., such that the polyisoprene obtained has a cis 1,4linkage content of greater than 99.0%, measured by carbon-13 nuclearmagnetic resonance and/or medium-wave infrared analysis.
 6. The processof claim 5 wherein the polymerization is carried out in the presence orabsence of an inert hydrocarbon polymerization solvent.
 7. The processfor the preparation of a synthetic polyisoprene according to claim 5,wherein the polymerization is carried at a temperature of −55° C. to−20° C., such that the polyisoprene obtained has a cis-1,4 linkagecontent which is equal to or greater than 99.3%.
 8. The process for thepreparation of a synthetic polyisoprene according to claim 5, whereinthe polymerization is carried out at a temperature of −55° C to −40° C.,such that the polyisoprene obtained has a cis-1,4 linkage content whichis equal to or greater than 99.5%.
 9. The process for the preparation ofa synthetic polyisoprene according to claim 5, wherein the rare earthsalt of the catalytic system is a rare earthtris[bis(2-ethylhexyl)phosphate].
 10. The process for the preparation ofa synthetic polyisoprene according to claim 9, wherein the rare earthsalt is neodymium tris[bis(2-ethylhexyl)phosphate].
 11. The process forthe preparation of a synthetic polyisoprene according to claim 5,wherein the catalytic system comprises the rare earth metal in aconcentration equal or substantially equal to 0.02 mol/l.
 12. Theprocess for the preparation of a synthetic polyisoprene according toclaim 5, wherein the catalytic system has a halogen donor:salt molarratio in a range from 2.6 to
 3. 13. The process for the preparation of asynthetic polyisoprene according to claim 5, wherein the catalyticsystem has a conjugated diene monomer:salt molar ratio in a range from25 to
 50. 14. The process for the preparation of a syntheticpolyisoprene according to claim 5, wherein the conjugated diene monomerof the catalytic system is butadiene.
 15. The process for thepreparation of a synthetic polyisoprene according to claim 5, whereinthe alkylating agent of the catalytic system is diisobutylaluminiumhydride.
 16. The process for the preparation of a synthetic polyisopreneaccording to claim 5, wherein the halogen donor of the catalytic systemis diethylaluminium chloride.